What kind of media are you using to clean the brass?
Possible that there was some media stuck in the flash hole, not enough to completely block things. Just enough to keep it from fully igniting. I have had it happen once before with a single round that had a piece of dirt clod in it that the tumbler missed. The decapping pin jut broke the through the middle is my guess it was not big. It caused the same problem for me that you described.
No matter how many times you do it and nothing happens it only takes something going wrong one time to kill you.